Look at the website to find all the attractions the 3 year old can go on. Decide if you'll be going on them. Decide if you want to wait in the standby line for them with the child or if you would prefer to get those attractions done faster. That tells if if EP is a good choice.
And decide if you'll indulge him/her in doing those attractions more than once per day. That will tell you if you should go with unlimited or just normal EP.